Hydraulic and pneumatic machines have long been known wherein work is performed by means of the application of a fluid under pressure to the base of a piston. As is known, such pistons are adapted for sliding movement within a cylinder and, in order to prevent fluid leakage around the edges of the piston, suitable sealing means are provided around the edge of the piston so as to form a fluid-tight seal between the piston and the cylinder. Typically, the sealing means is provided by an "O"-ring, accommodated within a corresponding recess around the periphery of the piston, such that the piston and combined "O"-ring form a fluid-tight seal within the cylinder.
Such an arrangement requires very accurate machining of the internal surface of the cylinder, since any flaws thereon would quickly abrade the "O"-ring, thereby destroying the seal and rendering the device at best inefficient and at worst, inoperative.
Additionally, in such known hydraulic and pneumatic pistons, there exists a high coefficient of friction between the "O"-ring and the internal surface of the cylinder, which friction manifests itself as a loss of energy in the form of heat and a consequent loss of efficiency of the machine.
Various solutions have been proposed for a so-called "frictionless" hydraulic piston, wherein a piston is adapted for sliding movement within a cylinder, an improved sealing means being provided which produces virtually no sliding friction throughout the piston's travel. U.S. Pat. No. 3,311,028 discloses the basic arrangement for a "frictionless" piston, incorporating a flexible, rolling diaphragm. The diaphragm is a sleeve of flexible material, e.g. rubber, one end of which fits over the piston and is sealed thereto. The other end of the sleeve is anchored around its external surface to the base of the hydraulic cylinder. As fluid is applied to the base of the piston within the cylinder, the piston rises, thereby causing the flexible sleeve to unroll. As force is applied to the piston from the other side, the piston falls, thereby causing the flexible sleeve to roll in upon itself such that the inner surface of the sleeve progressively faces outwards. Consequently, the rolling sleeve is referred to as a "flexible diaphragm".
Such an arrangement does indeed vastly reduce sliding friction between the piston and the internal cylinder wall. However, in order for the flexible diaphragm to be able to roll in on itself, as described, such that its internal surface progressively faces outwards, it must be formed of a material which is laterally compressible and, so far, this has demanded that relatively soft materials be employed therefor. The drawback of such an arrangement is that a high fluid pressure applied to the base of the piston is also applied to the internal surface of the flexible diaphragm, thereby causing the diaphragm to expand and, eventually, tear.
U.S. Pat. No. 3,438,309 describes a similar arrangement wherein there are additionally provided reinforcing means on the outer surface of the flexible diaphragm, which serve to prevent tearing of the diaphragm under the application of high hydraulic pressure. However, the proposed reinforcing means greatly restrict the movement of the piston, rendering it unsuitable for many practical applications.
A further drawback with many prior art systems concerns the fact that the rolling diaphragm itself is frusto-conical in shape in order that a tapering section of the diaphragm can roll easily into a wider section thereto. The use of such flexible diaphragms imposes a restraint on the stroke of the rolling piston in which it is used and this, in turn, severely constrains the resulting mechanical advantage of the hydraulic or pneumatic piston.
U.S. Pat. Nos. 2,849,026; 3,083,734; and 3,137,215, all in the name of J. F. Taplin, are directed to various methods for manufacturing rolling sealed diaphragms. Essentially, the methods disclosed by Taplin all rely on superimposing a flat, reinforcing membrane over a flat, flexible membrane and then moulding the two membranes together into a substantially frusto-conical hat-shaped or cylindrical diaphragm. Such diaphragms have a peripheral rim which is anchored between opposing flanges of a suitable cylinder. It will readily be apparent that rolling seal diaphragms produced in such manner are suitable for rolling pistons having a very limited stroke and are therefore subject to the same drawback concerning mechanical advantage as has already been explained.
Furthermore, since such membranes can only be manufactured by moulding, the manufacturing process is time-consuming and the resulting cost of each unit is relatively high.